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Regulatory assets and liabilities (Details) (USD $) | Dec. 31, 2013 | Dec. 31, 2012
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Regulatory assets and liabilities |  |  |  |
Regulatory assets: | $ 305,373,000 |  | $ 351,194,000 |
Regulatory liabilities: | 432,325,000 |  | 450,482,000 |
Net regulatory position | (126,952,000) |  | (99,288,000) |
Regulatory assets not earning a rate of return | 163,700,000 |  | 215,600,000 |
Plant removal and decommissioning costs (c) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 308,431,000 | [1] | 296,037,000 | [1]
Deferred income taxes |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 64,914,000 | [2] | 82,077,000 | [2]
Taxes refundable to customers (c) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 20,180,000 | [1] | 24,212,000 | [1]
Natural gas costs refundable through rate adjustments (d) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 16,932,000 | [3] | 35,328,000 | [3]
Other (c) (d) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 21,868,000 | [1],[3] | 12,828,000 | [1],[3]
Deferred income taxes |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | ** | [2] |  |
Regulatory assets: | 125,607,000 |  | 121,781,000 |
Pension and postretirement benefits (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | (e) | [4] |  |
Regulatory assets: | 105,123,000 | [5] | 166,477,000 | [5]
Taxes recoverable from customers (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Over plant lives | [6] |  |
Regulatory assets: | 18,266,000 | [5] | 9,078,000 | [5]
Manufactured gas plant sites remediation (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Up to 4 years | [6] |  |
Regulatory assets: | 15,797,000 | [5] | 15,828,000 | [5]
Natural gas costs recoverable through rate adjustments (b) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Up to 28 months | [6] |  |
Regulatory assets: | 12,060,000 | [7] | 2,981,000 | [7]
Long-term debt refinancing costs (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Up to 25 years | [6] |  |
Regulatory assets: | 8,697,000 | [5] | 9,144,000 | [5]
Costs related to identifying generation development (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Up to 13 years | [6] |  |
Regulatory assets: | 4,512,000 | [5] | 5,773,000 | [5]
Other (a) (b) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Largely within 1- 5 years | [6] |  |
Regulatory assets: | $ 15,311,000 | [5],[7] | $ 20,132,000 | [5],[7]
[1] | Included in other liabilities on the Consolidated Balance Sheets.
[2] | Represents deferred income taxes related to regulatory assets and liabilities. The deferred income tax assets are not earning a rate of return.
[3] | Included in other accrued liabilities on the Consolidated Balance Sheets.
[4] | Recovered as expense is incurred.
[5] | Included in deferred charges and other assets - other on the Consolidated Balance Sheets.
[6] | Estimated recovery period for regulatory assets currently being recovered in rates charged to customers.
[7] | Included in prepayments and other current assets on the Consolidated Balance Sheets.

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- Definition Estimated recovery period of regulatory assets. + References No definition available. + Details
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- Definition The amount of regulatory assets, excluding deferred income taxes, that were not earning a rate of return. + References No definition available. + Details
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- Definition The net amount of all regulatory assets less all regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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